The present invention relates generally to the removal of impurities contained in fluidic streams. More specifically, the invention relates to adsorbents for the removal of water and/or other oxygen-containing impurities from fluid streams comprising ammonia and methods for making and using same.
Ultra-high purity (UHP) ammonia (NH3) is widely used in a variety of different applications. For example, UHP ammonia may be used in the semiconductor industry for forming silicon nitride barrier layers in integrated circuits (IC). It is believed that the silicon nitride layers prevent metal migration during IC processing. Further, UHP ammonia is needed for manufacturing metal nitrides, such as gallium nitride, aluminum nitride, and indium nitride, that are used in light emitting diodes (LED) and laser diodes. These metal nitrides have the ability to emit light over a wide-spectral range. With the blue light capability, manufacturers of LEDs are now able to make these devices in any color of the spectrum. Furthermore, laser diodes are key components of optical storage media, see, e.g., P. Kung and M. Razeghi, Opto-electronics Review, 8(3) 201-239 (2000).
The electrical properties of devices such as IC devices and LEDs are very dependent on the impurity level in the nitride layers, which, in turn, is directly related to the purity of source ammonia used in manufacturing. Therefore, a purification system is normally required to remove impurities in ammonia. This purification is often done by passing a stream of gaseous or liquid ammonia through a purifier. One major target of the purifier is oxygen-containing impurities. Trace levels of oxygen-containing impurities in ammonia, such as water (H2O), oxygen (O2), carbon monoxide (CO), and carbon dioxide (CO2), can adversely affect production yields during the manufacture of semiconductor or other electronics devices as well as the end-product performance.
Among the oxygen-containing impurities, water may be the most challenging to remove. Water has a high affinity for ammonia due to its similar physical and chemical properties, e.g. molecular size and hydrogen bonding. For the manufacturing of semiconductor or other electronic devices, ammonia having water concentration at the ppm level is considered unacceptable. To ensure satisfactory performance of the manufactured device, the water content in the ammonia must be reduced to the ppb level, i.e., less than 100 ppb or below. Current drying methods and materials may have certain limitations when removing water and other oxygen-containing species down to the ppb level.
Ammonia has been traditionally supplied as a cylinder gas because the consumption rate of ammonia in these processes has been relatively low. Recently, however, the rapid growth in the LED market has increased the usage of ammonia thereby making it uneconomical to supply or use ammonia in this manner. Subsequently, the electronics industry is moving to “bulk supply” systems in which large storage vessels are used to supply ammonia. Using these systems, it would be more preferable to purify the ammonia at the storage vessel. This places new demands on the ammonia purification system. In this connection, the purifier must be capable of handling and purifying ammonia at relatively higher flow rates. Further, it is desirable for purifiers of bulk supply systems to have a longer useful life than point of use purifiers.
Bulk supply purification systems for removal of impurities from ammonia typically involve a purification bed containing sorbent media, scavengers, or adsorbents. These purification beds tend to be relatively large and the adsorbents contained therein are comparatively expensive. In order to reduce the operating costs and size of the system as well improve separation efficiency, it is desirable that the adsorbent satisfy one or more of the following criteria. First, the adsorbent should have relatively fast sorption kinetics since the efficiency of a purifier is directly related to the rate of sorption of the impurity on the media. Faster sorption rates may allow for smaller sorption beds thereby increasing the purification efficiency at a lower cost. Second, the adsorbent should have relatively high sorption capacity since the size of the purifier is directly related to the sorption capacity of the adsorbent. Adsorbents with high capacity are thus required to reduce the overall purifier size and cost. In addition, a relatively high adsorption capacity at the water partial pressure range of interest, up to 6.65×10−3 torr, is necessary to reduce the impurity levels of ammonia to the ppb level. Third, the adsorbent should be stable in ammonia to prevent the formation of gaseous or volatile by-products generated during purification. Fourth, the adsorbent should be nonvolatile at the temperatures needed for purification. Lastly, the adsorbent is preferred to be regenerable.
There are a number of physical and chemical adsorption methods in the prior art for removing water and other oxygen-containing impurities from ammonia. One chemical adsorption method to remove water from ammonia involves metal oxide adsorbents. For example, Japanese Patent 97142833 discloses removal of water from ammonia by contacting the gas with an adsorbent comprising BaO, or a mixture containing BaO as the major compound, whereby water is removed through a chemical reaction with the metal oxide. Because the major mass transfer limitation may be through the reaction product, this approach may suffer from low adsorption kinetics.
Another chemical adsorption method is described in EPs 0484301 B1 and 0470 936 B1. These patents describe the use of hydrogenated getter metal alloys comprised of varying amounts of zirconium (Zr), vanadium (V) and iron (Fe) with a preferred composition of 70% Zr, 24.6% V, and 5.4% Fe. These hydrogenated getter metal alloy adsorbents are impractical for bulk purification of ammonia for several reasons. The preparation of these alloys involves multiple steps prior to use: the alloys need to be activated by heating to elevated temperatures of around 350° C. in a reducing gas stream and the alloys need to hydrogenated, or treated in a stream of hydrogen. Further, the alloys may require an operating temperature of over 100° C. to work properly.
Yet another chemical adsorbent method is disclosed in U.S. Pat. No. 6,241,955. The '955 patent discloses an adsorbent that is a reduced metal oxide, solid substrate having a surface area of 100 m2/g or larger. An oxide, such as manganese or molybdenum oxides, is partially reduced in H2 or another agent to produce active sorption sites. It is believed that the gaseous contaminants are removed by a combination of reaction with metal active sites and adsorption on the substrate surface.
A still further example of a chemical adsorption method is disclosed in European patent application EP 1,176,120. The '120 application describes removing water and other impurities from ammonia by contacting the ammonia with an adsorbent having manganese oxide and/or nickel oxide as an active ingredient on a porous support. The adsorbent is prepared by reduction in hydrogen of the metal oxide at temperatures greater than 500° C. for manganese and up to 350° C. for nickel. The ammonia may further be passed through a bed of synthetic zeolite.
Further examples of prior art, chemical adsorbents consist of a scavenger deposited on an organic support. For example, U.S. Pat. No. 4,761,395 discloses metallic carbanion or anion scavengers on an organometallic support. The active scavenger sites are formed by the reaction between a protonated carbanion and an organic deprotonating agent. A major concern of using this approach is the possibility of releasing hydrocarbon impurities to the purified gas stream if the organic deprotonating agent is not fully removed from the support or if the protonated carboanion is an organic material. Hydrocarbon contamination, even at very low levels, may be deleterious to semiconductor devices. Yet another example of a scavenger on an organic support is found in U.S. Pat. No. 5,531,971. The '971 patent discloses a pyrolyzed metal scavenger deposited on a polymeric or macroreticulate polymer support. The pyrolyzed metals are selected from Group IA of the Periodic Table. One potential problem with this approach is that the scavenger may release metal particles into the purified gas stream. Further, the reaction between free alkyl metal and water may release gaseous H2. The introduction of new impurities from either of the adsorbents described in the '395 or '971 patents, namely hydrocarbons, metal particles, and gaseous H2, is unacceptable in the manufacturing of semiconductor, LED, or other electronic devices.
Further examples of prior art chemical adsorption methods involve scavengers deposited on an inorganic support. In this connection, published application WO 00/23174 describes a scavenger that comprises an active agent on an inorganic support such as a zeolite, alumina, or silica material. The active agent is formed by pyrolysis of an adsorbed hydride at an elevated temperature. Because of the limited number of function groups on the surface of an inorganic support and the unfavorable pyrolysis reaction, the amount of active sites may be limited.
A still further example of a prior art adsorption method is the use of CaSO4 by itself to remove water from ammonia. One drawback to this approach is the low achievable water adsorption efficiency due to its limited surface area.
The aforementioned chemical adsorption methods are typically not regenerable because the reaction between the active phase of the scavenger and water is almost irreversible. To remedy this, physical adsorption on molecular sieves may be used to remove water from ammonia. This method, however, may be inefficient because of the thermodynamic properties of ammonia and water. Since their thermodynamic properties are similar, ammonia and water may compete for the adsorption sites on the material.
Accordingly, there is a need in the art to provide new adsorbents to purify ammonia to the ppb level. There is a need in the art to provide regenerable adsorbents that have a relatively high sorption capacity and relatively faster sorption kinetic. There is also a need in the art to provide adsorbents that operate effectively at ambient temperatures. Further, there is a need in the art for adsorbents that can avoid the introduction of additional contaminants into ammonia during the purification process. Moreover, there is a need in the art for processes for making adsorbents that require fewer process steps to manufacture and have lower activation temperatures.
All references cited herein are incorporated herein by reference in their entirety.